Voting got off to a rough start in volatile Haiti as angry mobs stormed voting centres that failed to open on time, with one person dying of a heart attack and another of asphyxia.

Several more people were injured or fainted as they were trampled or shoved by crowds that rushed voting offices.

Many voters rose well before dawn, walked for several hours only to wait in long lines to cast their ballot in the first election since Jean Bertrand Aristide, the former president, fled the violence-wracked country two years ago. There were no reports of violence overnight.

"We had a very calm night, contrary to past election eves in Haiti," said Juan Gabriel Valdez, the UN special representative to Haiti.

But tension remained high in a country terrorised by armed gangs, plagued by rampant poverty, and with a history of fraudulent elections and military coups.

A man died of asphyxiation and another of a cardiac arrest, according to a senior UN official who monitored the voting.

"It's because of the long wait," he said, asking not to be identified.

Anger mounted among the massive crowds waiting outside voting centres, many of which remained closed several hours after the balloting officially started.

Tension was particularly high around the notoriously violent Cite Soleil slum, where voters voiced their anger chanting "open up, open up."

High alert

The 9500-strong UN military and police force, which has been in Haiti since Aristide resigned and left for exile, stepped up security following an explosion of violence in recent months.

The voting was initially scheduled for November, but was postponed four times as UN forces battled armed gangs and officials struggled to train electoral staff and organise voting materials for the 3.5 million registered voters.

In the dirt-poor slums of the capital that have been the flashpoint of the violence, many are backing Rene Preval, 63, a former president, who has long supported Aristide and is the clear frontrunner in opinion polls.

Aristide was widely popular among impoverished Haitians, who make up 77% of the 8.5 million population and who often blame the United States, France and Canada for his departure.

"We hope Preval will win, we want things to change at all levels - jobs, education, stability," said Clairmont Ronald, 35 and unemployed, as he waited in a long line to cast his ballot.

Preval draws little support among better-off Haitians, who seem to favour Charles Henry Baker, 50, an industrialist or Leslie Manigat, 75, former president.

"We should not be afraid of change, we should return to the old ideas," said businessman Alex Turner, 53, who said he would vote for Baker, but would wait till the crowd thins out.

Opinion polls

Opinion polls ahead of the election gave Preval a lead of at least 27% over Baker and Manigat, but the surveys' reliability was uncertain.

Should no candidate obtain 50% of the vote, the frontrunner would face off with the second placed candidate in March.

Among the longshot candidates is Guy Philippe, 37, a former police commissioner who led the insurgency that played a key role in pushing out Aristide two years ago.

More than 800 voting centres have been set up for the election that will also renew the 130-seat legislature.

Protectively, schools have been closed, government offices will be shuttered until Wednesday and American Airlines suspended Monday and Tuesday's flights to Haiti.